1. Field of the Invention
The present invention relates to an oscillating circuit. More particularly, the present invention relates to a variable frequency oscillating circuit having broad variable range frequency application, excellent high frequency characteristics, and improved frequency selectivity.
2. Description of the Art
Advances in semiconductor fabrication techniques have drastically changed the way designers approach circuit design problems. Highly complex analog/digital circuits can now be formed on large scale semiconductor integrated circuits. As a result, problems of reliability and device stability are substantially mitigated by mass produced integrated circuit component substitutes which are significantly more miniature and cheaper to manufacture.
Signal-processing systems formed almost entirely on a single integrated circuit chip have been successfully used in connection with audio systems and video cassette recorders. Such signal-processing systems consist essentially of a phase locked loop (PLL) circuit capable of providing stable signal processing using highly reliable circuit components fabricated on a single integrated circuit.
Phase locked loop (PLL) circuits having a voltage controlled oscillator (VCO) included therewith are common. One well known voltage controlled oscillator (VCO), in particular, comprises a band pass filter and a positive feedback amplifier and is shown in FIG. 1. This VCO was described in an article in IEEE Transactions on Consumer Electronics, Vol. 35, No. 4, pp. 744-745, published November 1989.
In the voltage controlled oscillator (VCO) of prior art FIG. 1, there is provided a band pass filter BPF 1 and feedback amplifier 4. This circuit is designed to oscillate at an intermediate frequency of the BPF, the intermediate frequency controlling an output voltage.
BPF 1 comprises variable transconductance (gm) amplifiers 2 and 3 and condensers C.sub.0, as shown in FIG. 1. Feedback amplifier 4 comprises resistance R and a voltage-current converter 5. Voltage-current converter 5 acts as a current mirror to selectively output variable current I.sub.0 in response to variable voltage V.sub.0.
Variable voltage V.sub.0 is output from BPF 1 and coupled as a feedback signal to voltage-current converter 5. Variable voltage V.sub.0 output from BPF 1 serves as an output of the voltage controlled oscillator and is coupled to a phased locked loop (PLL) circuit (not shown).
Variable voltage V.sub.0 can be expressed by the following equation: ##EQU1## where w.sub.0 =gin/C.sub.0 and Q.sub.0 =1/(gmR) . Also, s=jw.sub.0 =j2.pi.f.sub.0 --
for example, condenser impedance EQU z.sub.c =1/2.pi.f.sub.c =1/s.sub.c
Load resistor R serves as a damper to BPF 1, the resistance value selected to control the selectivity Q.sub.0 of the band pass filter. It is important to select a low value load resistance R in order to achieve high selectivity. Similarly, a low value load resistance results in greater frequency stability during VCO oscillation as it supplies the variable voltage V.sub.0 as an input to feedback amplifier 4. Oscillating frequency f.sub.0 can be derived from equation (1) above as: ##EQU2## where EQU f.sub.0 =w.sub.0 /2.pi..
Frequency f.sub.0 varies with the voltage-controlled current I.sub.x. When the frequency corresponds with the predetermined intermediate frequency, current I.sub.x shifts to compensate for any variations in impedance. Thus, I.sub.x is variable in order to adjust frequency f.sub.0 to the necessary value.
A stable oscillation requires a definite amplitute of output voltage V.sub.0. When oscillation is at a frequency f.sub.0, equation (1) can be rewritten as: ##EQU3##
If current I.sub.0 from voltage-current converter 5 in feedback amplifier 4 is designed to be proportional to variable current I.sub.x, the amplitude of output voltage V.sub.0 remains constant even if control voltage V.sub.CTL changes.
A transconductance value for each of variable transconductance amplifiers 2 and 3 in the circuit of prior art FIG. 1 can be derived as a function of the ratio of control voltage V.sub.CTL to feedback current I.sub.0.
The above described voltage controlled circuit of the prior art FIG. 1 has the following problems.
A positive feedback principle of operation is necessary so as to stably maintain a variable frequency output therefrom. As a result, active elements forming the variable transconductance (gm) amplifiers 2 and 3 operate in a non-linear operating region of a characteristic curve, unstably affecting the operating characteristics of the respective amplifiers and resulting in instability of the signal waveform output therefrom.
Because in this particular case, transconductance value (gm) is a function of frequency, unstable gm values render inoperable a wide portion of the variable range of the oscillating frequency of the VCO and reduce stability in the high frequency spectrum due to deterioration in signal response.
Large differences thus exist between desired transconductance values and transconductance values otherwise generated by variable current I.sub.0 from the positive feedback system.
Higher harmonics of distorted waveform are also present in the prior art circuit resulting in unwanted noise in the input waveform of adjoining circuits coupled thereto.